Structure

Where things live: one project's data, what registration does to it, and the settings that belong to the machine rather than to any run.

Project data	the data contract: what a dataset is, and the canonical path it takes under $MD_DATA
Registered data	what registration moves, what it leaves behind as a symlink, and why the source directory still works afterwards
Machine configuration	user.config: identity, $MD_DATA, platform, precision and device policy